1. Field of the Invention
The present invention relates to a golf ball and more particularly to an improvement in a dimple of the golf ball.
2. Description of the Related Art
A golf ball has approximately 300 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight of the golf ball to accelerate the transition of a turbulent flow over a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backwards so that a pressure resistance is reduced, resulting in an increase in a flight distance of the golf ball. Moreover, the acceleration of the transition of the turbulent flow increases a distance between upper and lower separating points of the golf ball which is generated by a backspin. Consequently, a lift acting on the golf ball is increased. A dimple capable of easily accelerating the transition of the turbulent flow, that is, a dimple capable of better disturbing an air stream is more excellent in an aerodynamic characteristic.
In order to enhance the aerodynamic characteristic, there have been various proposals for the shape of a dimple. U.S. Pat. No. 5,890,975 has disclosed a golf ball having an elongated dimple. Moreover, Japanese Unexamined Patent Publication No. 2000-185113 has disclosed a golf ball having a dimple taking an elliptic planar shape (the contour shape of the dimple seen in the direction of a normal), a golf ball having a dimple taking an oval planar shape, and a golf ball having a dimple taking a planar shape of a teardrop.
A golf player is very interested in making a good score and causing a golf ball to fly to a distance. A large number of golf players desire a golf ball which is more excellent in flight performance. However, the golf ball to meet the demand of the golf player has not been obtained.
In consideration of the circumstances, it is an object of the present invention to provide a golf ball which is excellent in flight performance.
In order to attain the above-mentioned object, the invention provides a golf ball having, on a surface thereof, a large number of non-circular dimples taking planar shapes other than a circle and a large number of circular dimples taking circular planar shapes,
wherein a ratio of the number of the non-circular dimples to the number of all dimples is 77% to 92%,
a ratio of the number of the circular dimples to the number of all the dimples is 8% to 23%,
the non-circular dimples include plural kinds of dimples taking specific shapes,
a ratio of the total number of the dimples taking specific shapes to all the non-circular dimples is 80% or more,
when the number of a certain kind of dimples which contour line length is minimum in kinds having a specific shape and a dimple number ratio of 5% or more is represented by Ns and the number of a certain kind of dimples which contour line length is maximum in kinds having a specific shape and a dimple number ratio of 5% or more is represented by Nb, a ratio (Ns/Nb) is 20/80 to 80/20, and
a dimple pattern is rotation symmetrical at an angle of 120 degrees or 90 degrees by setting a pole to be a center of rotation in both of two hemispheres obtained by a division through a parting surface of a mold.
The dimples taking specific shapes satisfy the following conditions (A) and (B):
(A) a contour line in a planar shape is formed by a combination of a semicircle and a circular arc or a combination of a semicircle and an elliptic arc; and
(B) the contour line does not have a convex portion therein.
In the golf ball, the multiplied effects of the dimple taking the specific shape and the circular dimple can enhance an aerodynamic characteristic. The golf ball is excellent in flight performance.
It is preferable that the dimple pattern should not be mirror symmetrical in both of the two hemispheres obtained by the division through the parting surface of the mold. In other words, even if the hemisphere is further divided into two portions on any plane to be a quarter sphere, the dimple patterns of two quarter spheres are not symmetrical with respect to the plane. Consequently, the dimple effect can further be enhanced.
It is preferable that the surface of the golf ball should be provided with twelve or more specific land portions surrounded by five dimples taking a specific shape which are arranged at a substantially equal center angle pitch. By the specific land portion, the disturbance of the air is further promoted.
It is preferable that the specific land portion should be provided uniformly on a surface. Consequently, almost equivalent dimple effects can be obtained irrespective of the position of a portion where the circumferential speed of a backspin is the highest (which will be hereinafter referred to as xe2x80x9cthe highest speed portionxe2x80x9d).
It is preferable that a sum of volumes of dimples (in a portion surrounded by a virtual sphere of the ball and surfaces of the dimples) is 400 mm3to 550 mm3. Consequently, it is possible to obtain more excellent flight performance.
The present invention will be described below in detail based on preferred embodiments with reference to the drawings.